I testify that there is No God but Allah, alone without partners, and that Muhammad is His slave and Messenger.

This is based on these two hadith from Sahih Muslim

The Book of Purification (Kitab Al-Taharah)
Muslim :: Book 2 : Hadith 451

‘Uqba b. ‘Amir reported: We were entrusted with the task of tending the camels. On my turn when I came back in the evening after grazing them in the pastures, I found Allah’s Messenger (may peace be upon him) stand and address the people. I heard these words of his: If any Muslim performs ablution well, then stands and prays two rak’ahs setting about them with his heart as well as his face, Paradise would be guaranteed to him. I said: What a fine thing is this! And a narrator who was before me said: The first was better than even this. When I cast a glance, I saw that it was ‘Umair who said: I see that you have just come and observed: If anyone amongst you performs the ablution, and then completes the ablution well and then says: I testify that there is no god but Allah and that Muhammad is the servant of Allah and His Messenger, the eight gates of Paradise would be opened for him and he may enter by whichever of them he wishes.

The Book of Purification (Kitab Al-Taharah)
Muslim :: Book 2 : Hadith 452

Uqba b. ‘Amir al-Juhani reported: Verily the Messenger of Allah (may peace be upon him) said and then narrated (the hadith) like one (mentioned above) except (this) that he said: He who performed ablution and said: I testify that there is no god but Allah, the One, there is no associate with Him and I testify that Muhammad is His servant and His Messenger.

From these two hadith, we learn that
1. Say the shahadah after performing the wudhu
2. Perform 2 rakaah sunnah prayer after wudhu’
3. The reward is paradise.
4. And specifically for those who recite the syahadah, the reward is he can enter whichever eight gates of Paradise He wishes. May we be among these people.

Also on top of the shahadah there is a dua that we can recite. This dua is based on the Saheeh Tirmidhi